1. Field of the Invention
The present invention relates to a liquid crystal display device having color compensation in a supertwisted liquid crystal.
In this specification, a phase difference plate means a retardation plate or a birefringence plate.
2. Description of the Prior Art
Generally, a supertwisted nematic liquid crystal display device (STN-LCD) is colored in yellow-green or blue, but a bright and sharp black/white display is obtained by using a color correction plate. As a result, the display quality is enhanced. Further, it may be used in the word processor, computer and other automated equipment.
In such a color-compensated two-layer STN-LCD, the coloration produced in the first layer (the cell for driving) is corrected in the second layer (the cell for optical compensation) to turn into a colorless display. This structure requires two liquid crystal cells, as compared with the single-layer STN-LCD, and the thickness and weight of the display device are increased.
On the other hand, in the phase difference plate type STN-LCD, it is known to dispose a phase difference plate before the liquid crystal cell, and to dispose one plate, each at the front side and back side of the liquid crystal cell. However, as compared with the two-layer type STN-LCD, the contrast is inferior and a sufficient display quality is not obtained (for example, the Japanese Laid-open Patent 64-519).
According to the Japanese Laid-open Patent 64-519, phase difference plates are disposed at the front side and back side of the STN liquid crystal panel. In its Embodiment 21, the sum of retardations of the two is about 0.6 .mu.m (600 nm). However, nothing is mentioned about the individual values. Incidentally, when the present inventors attempted to dispose the 300 nm phase difference plates in the system disclosed in Embodiment 21, a satisfactory black/white display was not obtained.
A phase difference plate is also called a retardation plate or a birefringence plate.